A modular multilevel converter (MMC) comprising an on/off-controllable switching device, such as an insulated gate bipolar transistor (IGBT), is a converter capable of outputting a voltage exceeding a breakdown voltage of the on/off-controllable switching device. The MMC is promising in applications, such as a high voltage DC power transmission system (HVDC), a static compensator (STATCOM), and a motor drive inverter.
[Non Patent Literature 1] discloses a circuit system for the MMC.
According to Non Patent Literature 1, the MMC comprises a plurality of unit-converters (hereinafter, referred to as cells) serially-connected (cascaded). Each cell is a bidirectional chopper circuit, for example, and includes a switching device and a DC capacitor. Each cell is coupled to the outside via at least two terminals, and can control the voltage between the two terminals to a voltage of the DC capacitor which this cell includes, or to zero.
In PWM-control of each cell, by appropriately shifting the phase of a triangular waveform carrier supplied to each cell, the output voltage waveform of the MMC can be formed into a multilevel waveform. This can reduce the harmonic component as compared with the case of a two-level converter.
The features of the MMC include that the potential of each cell differs from each other and that there is a cell with a high earth potential. In particular, when the MMC is applied to HVDC or the like, the earth potential of a cell reaches several tens of kV to several hundreds of kV. In addition, the earth potential of each cell momentarily varies.
However, in Non Patent Literature 1, an isolation voltage between a controller and each switching device is not considered because the device at a laboratory level is targeted at.
Non Patent Literature 2 discloses a configuration, wherein a signal processing circuit with the same potential as each cell is mounted in the vicinity of the each cell, and wherein a central controller at the earth potential and each signal processing circuit are coupled to each other by an optical fiber cable.